-NextDNS is a product with a free tier. It will always be limited in that sense.

+Pihole is free and open. It is also yours to build,manage,customize as you please.

-NextDNS is also further away, meaning there will be much more latency for all your DNS queries. It is usually best to run your own resolver, or have a local DNS server in your network.

+Pihole sits on a device on your network. You can also enable recursion directly on the pihole by installing Unbound on the same device.